1. Field of the Invention
This invention relates to a control system for hybrid vehicles having an internal combustion engine and a traction motor as prime movers.
2. Prior Art
Conventionally, a hybrid vehicle having an internal combustion engine (hereinafter simply referred to as "the engine") and a traction motor as prime movers is widely known, and a control system for controlling the prime movers of such a hybrid vehicle has already been proposed e.g. by Japanese Patent Laid-Open Patent Publication (Kokai) No. 5-229351.
The proposed control system determines the optimum torque at which the maximum engine efficiency is attained, in dependence on traveling conditions of the vehicle, and at the same time detects actual torque generated by the engine for actually driving the vehicle. Then, the control system determines or selects demanded or required torque from the optimum torque and the actual torque. When the optimum torque is selected as the demanded torque, and at the same time, the optimum torque is larger than the actual torque, regenerative current is caused to be generated for regeneration of energy (specifically, electrical energy).
However, the conventional control system does not control an output from the traction motor in dependence of the state of charge (amount of remaining charge) of an electrical storage battery which supplies electric power to the traction motor to drive the same. As a result, there is a possibility of a sudden drop in the remaining charge of the battery during traveling of the vehicle. Further, the proposed control system does not optimally control the balance of the assistance of the traction motor to the engine by providing torque thereto and the engine output. As a result, it is difficult to control the operation of the hybrid vehicle so as to improve both the fuel economy and power performance of the hybrid vehicle.